Prevent calling count() on null object
Prevent calling count() on null object

Calling count on the null object is throwing errors like:
`PHP Fatal error:  Method Khill\FontAwesome\FontAwesome::__toString() must not throw an exception`

(On PHP 7.2 anyway)

Adding `!empty()` ensures it doesn't attempt to iterate over the null object.
kevinkhill/FontAwesomePHP